There's been A LOT of discussion on Japanese sites about how to unlock
the following hidden characters and who the last hidden character is.
Some people say it's the silhouette of Rauzaruku Gash who can be obtained
by getting a 27th item, the Glowing Red Book. No one's managed to figure
out the exact way of unlocking the last character yet from the looks of
things.

If you want to get through Challenge Mode quick, you can set the number of
wins in options to 1.

Brago & Berserk Sherie
----------------------

Defeat them twice in Challenge Mode on normal or hard with Gash. You can
find their translated dialogue in the "Story Mode" section of this guide.

Moves are just the same as normal Sherie from above.

Zeon & Dufour
-------------

Have 15 items (approx. 58%) in your collection and defeat Challenge Mode
winning ALL rounds with a KO using Gash. It seems Zeon can appear with
other characters too so you may not have to use Gash only.

Refer to "Story Mode" to view the dialogue.

Umagon & Raiku Sensei
---------------------

There's 3 things you must to do unlock this duo. First you must have
the Umagon Toy. Once you have that, the time limit MUST be set to 60
seconds and then you must beat ALL opponents after the time runs out
in Challenge Mode using Gash.

Refer to "Story Mode" to view the dialogue.

??? & ???
---------

No one seems to know how to unlock this duo yet. This should be in the next
update once I find out.